This invention relates to an apparatus and method for attenuating unwanted narrow band noise, and more particularly to a method and apparatus for tracking and attenuating a narrow band noise signal which drifts in frequency, while minimizing the delay of the filtered signal.
Airframe-mounted solid-state sensors are used with aircraft to generate a body-rate signal for attitude control in a flight-control system. This body-rate signal is sometimes contaminated by a large narrow band-noise signal caused by a resonant mode within the sensor being driven by a wide band vibration noise. The Q of the resonant element can be on the order of several thousand. Simple low pass or band pass filtering cannot be used to eliminate this narrow band noise because such filtering introduces excessive phase shift in the flight-control channel. If the frequency range of this noise could be restricted to a couple of hertz, then a fixed, narrowband notch filter could provide adequate attenuation without introducing excessively large phase shift. Unfortunately, the frequency deviation is not always very small with such narrow band noise signals. This is in part because the resonant frequency of these signals is temperature and time dependent. The resonant frequency may also be affected by variations and atmospheric pressure and other factors affecting operation of the sensor.
There presently is not a solution to the problem of attenuating narrow band noise signals which vary significantly in frequency. Fixed narrow band notch filters have been used successfully to attenuate narrow band noise when the center frequency of the noise has been tightly controlled. By opening the specifications on the noise center-frequency variance, more sensors are able to pass acceptance tests, thereby lowering unit costs. The acceptable temperature ranges of the sensor can also be expanded because the frequency of the narrow band-noise signal is temperature sensitive. However, previously developed systems still have not been able to successfully track and attenuate a narrow band noise signal which varies significantly in frequency while minimizing delay or phase shift in the signal path.
Previously developed systems have suffered from a variety of other drawbacks. Some presently available systems work well for sinusoidal noise signals but not for random narrow band noise signals. Some systems require a more fully or distinctly specified noise. Additionally, some systems have implemented an adaptive filter, but the functioning of the filter has been limited unless the statistics of the noise met certain prescribed criteria. With other filtering systems, the necessary performance requirements (notch depth, tuning bias, etc.) were simply not achievable.
Accordingly, it is a principal object of the present invention to provide an adaptive resonance canceller system and method for tracking and attenuating unwanted narrow band noise within a large frequency spectrum, and without introducing significant phase shifting of the filtered signal.
It is yet another object of the present invention to provide an adaptive filter for attenuating large narrow band noise signals present in an output signal of a solid-state sensor, wherein the narrow band noise signal varies significantly in frequency, and where a notch filter for removing the narrow band noise signal is tuned, in real time, such that it tracks the center frequency of the narrow band noise signal to thus enable attenuation of the noise signal, and further without introducing significant phase shift of the sensor output signal.
The present invention relates to an adaptive resonance canceller system and method for attenuating a large narrow band noise signal within a sensor output signal, where the narrow band noise signal may vary significantly in frequency. The system of the present invention generally comprises a notch filter for removing the unwanted narrow band noise signal from the sensor output signal. The sensor output signal is further input to an error reference and gradient generator which generates an error reference signal and an error gradient signal. These signals are input to a complex correlator system which produces a tuning parameter signal for precisely tuning the notch filter to the center frequency of the narrow band noise signal. This tuning parameter signal is applied to the notch filter to thus tune the notch filter, in real time, to a center frequency of a narrow band noise signal such that the filter can attenuate the noise signal. Advantageously, this action is performed without introducing any tangible phase shift into the output signal of the sensor. The tuning parameter signal is also applied, in closed loop fashion, to the error reference and gradient generator such that the error reference and error gradient signals applied to the complex correlator system can be modified in accordance with the changing frequency and magnitude of the narrow band noise component of the sensor output signal.
In the preferred embodiment, the output signal from the notch filter has a frequency spectrum which is substantially identical to that of its input signal (i.e., the sensor output signal) but without the narrow band noise component being present. The notch filter comprises a gain scaling multiplier, a biquadratic allpass filter and an adder. In the preferred embodiment the biquadratic allpass filter comprises a well known, second order, recursive, single-multiplier-per-order Gray-Markel allpass lattice filter whose design has been optimized for L1 scaling.
The error reference and gradient generator comprises a bandpass filter, an automatic gain control, a scaling element, an allpass filter, an adder, a subtractor, and a delay element. The bandpass filter frequency is selected to have a passband range to accommodate the expected maximum and minimum frequencies of the narrow band noise component of the sensor. The scaling multiplier, the allpass filter, the adder and the subtracter are used to produce from the output signal of the automatic gain control the error reference and error gradient signals which are applied to the complex correlator system.
The complex correlator system is used to receive the error reference and error gradient signals and to produce therefrom the tuning parameter signal needed to tune the notch filter to a center frequency of the narrow band noise component. Essentially, the complex correlator sums the product of the error reference and error gradient signals with the product of the Hilbert transform of the error reference and Hilbert transform of the error gradient signals to produce a strong error signal which is then scaled, applied to an accumulator (i.e., integrator), then further scaled and biased to produce the tuning parameter signal.
The apparatus and method of the present invention thus provides a means for tracking and attenuating a narrow band noise component of an input signal, where the narrow band noise component varies significantly in frequency. Importantly, the apparatus and method of the present invention performs its function without introducing tangible phase shift of the input signal. As such, the method and apparatus of the present invention is particularly well adapted for use in filtering the output signals of sensors used in airframe mounted sensors where the resonant frequency of the sensor output is temperature and time dependent, and where the filtering of the sensor output signal must be performed without introducing significant phase shift. The present invention also provides a means to isolate a narrowband noise of a signal for purposes of analysis, and without resorting to spectrum analysis methods which would require other problems such as bin resolution, window shaping, etc. to be addressed.